The present invention relates to a multilayered gas sensing element capable of detecting a specific gas (such as NOx) concentration and preferably employable in an exhaust system for an internal combustion engine of an automotive vehicle.
Harmful gases emitted from automotive internal combustion engines cause serious air pollution the modem society now faces. Various laws and regulations require automotive manufacturers to satisfy severe standards for promoting purification of emission gases. Under such circumferences, it is known that the emission gas purification can be effectively performed by directly detecting the NOx concentration to feedback control the engine combustion as well as to monitor the catalyst condition based on the detected NOx value.
FIG. 17 shows a conventional multilayered gas sensing element 9 which is installable in the exhaust system of an automotive internal combustion engine and is capable of detecting the NOx concentration in the exhaust gas.
The multilayered gas sensing element 9 comprises solid electrolytic sheets 963 and 965 and insulating sheets 964 and 966 stacked to form a first chamber 911 and a second chamber 912 into which an objective gas to be measured is introduced.
A first diffusion resistive passage 910 connects the first chamber 911 to an outside of the gas sensing element 9. A second diffusion resistive passage 920 connects the first chamber 911 to the second chamber 912. Furthermore, the multilayered gas sensing element 9 comprises a reference gas chamber 914 into which a reference gas is introduced.
A first electrochemical cell 902, located or provided in the first chamber 911, pumps in and out oxygen in accordance with an applied voltage. A second electrochemical cell 903, located or provided in the second chamber 912, is responsive to application of a predetermined voltage for generating a sensor current representing a NOx concentration in the objective gas.
The second electrochemical cell 903 has an electrode on which NOx is reducible. Therefore, the NOx concentration can be measured based on an ion current caused by the reduction of NOx.
Accordingly, when excessive oxygen resides in the second chamber 912 and the oxygen concentration is fluctuating, the second electrochemical cell 903 produces an sensor output corresponding to a sum of NOx concentration and fluctuating oxygen concentration in the second chamber 912. Thus, the NOx concentration detected by the second electrochemical cell 903 is inaccurate.
To solve this problem, the first electrochemical cell 902 is provided on the surface of the first chamber 911. The first electrochemical cell 902 discharges oxygen from the first chamber 911 to the outside of the gas multilayered sensing element 9 so as to maintain the oxygen concentration in the first and second chambers 911 and 912 to a constant level.
However, according to the above-described conventional multilayered gas sensing element, when the air-fuel ratio of the exhaust gas to be measured is shifted to the rich side, it is necessary to introduce oxygen into the first chamber 911 to oxidize the reducible gas, such as propane, contained in the measured exhaust gas. However, one of the electrodes of the first electrochemical cell 902 is not brought into contact with the reference gas in the reference gas chamber 914. This makes it impossible to detect the air-fuel ratio being switched to the rich side. Hence, the first electrochemical cell 902 cannot function properly in the rich side of air-fuel ratio.
Furthermore, in this case, the inside of second chamber 912 is filled by a rich environment. An inverse electromotive force is applied to the second electrochemical cell 903. This makes it impossible to detect the NOx concentration.
Similar problem will arise in general multilayered sensing elements having a sensing mechanism for detecting a specific gas concentration based on an ion current caused in response to oxygen ion caused when the specific gas is decomposed on the electrode of the second electrochemical cell.
To solve the above-described problems, an object of the present invention is to provide a multilayered gas sensing element capable of accurately detecting a specific gas concentration of an exhaust gas emitted from an automotive engine even when combustion condition changes so widely that the air-fuel ratio changes in a wide range from a lean side to a rich side.
In order to accomplish the above and other related objects, the present invention provides a multilayered gas sensing element comprising first and second chambers into which an objective gas to be measured is introduced, a first diffusion resistive passage connecting the first chamber to an outside of the gas sensing element, a second diffusion resistive passage connecting the first chamber to the second chamber, a first electrochemical cell having one end located in the first chamber and the other end provided in a reference gas chamber for pumping in and out oxygen in accordance with an applied voltage, and a second electrochemical cell having one end located in the second chamber and being responsive to application of a predetermined voltage for generating a sensor current representing a specific gas concentration in the objective gas.
The multilayered gas sensing element of the present invention is characterized in that the first electrochemical cell has one end located in the first chamber and the other end provided in the reference gas chamber and the second electrochemical cell has one end located in the second chamber.
The multilayered gas sensing element of the present invention functions in the following manner.
The first electrochemical cell faces both the first chamber and the reference gas chamber.
When the air-fuel ratio is shifted to the rich side, such switching of air-fuel ratio can be detected as the one end of the first electrochemical cell is located in the reference gas chamber. Accordingly, the first electrochemical cell can pump oxygen into the first chamber.
Furthermore, pumping operation of the first electrochemical cell makes it possible to maintain the oxygen concentration in the second chamber to a constant level. Accordingly, no inverse electromotive force is applied to the second electrochemical cell. This makes it possible to measure a specific gas concentration of the measured gas in a wide range from the lean side to the rich side.
As apparent from the foregoing, the present invention provides a multilayered gas sensing element capable of accurately detecting a specific gas concentration in a measured gas exhausted from an internal combustion engine even if combustion condition is controlled with the air-fuel ratio varying in a wide range from the lean side to the rich side.
Furthermore, according to the multilayered gas sensing element of the present invention, the oxygen pumping in and out operation causes an oxygen ion current flowing in an electric circuit of the first electrochemical cell.
The oxygen ion current has a current value representing the air-fuel ratio. Therefore, the first electrochemical cell can be used as an air-fuel ratio detecting cell.
In other words, the multilayered gas sensing element of the present invention can measure both the specific gas concentration and the air-fuel ratio simultaneously.
Application of the multilayered gas sensing element of the present invention is not limited to measurement of NOx concentration. Therefore, the multilayered gas sensing element of the present invention is applicable to other types of gas sensors, such as a CO sensor and a HC sensor.
Furthermore, according to the present invention, it is preferable that the second electrochemical cell has the other end located in the reference gas chamber.
This arrangement is effective to stabilize an electric potential of the positive electrode side of the second electrochemical cell. Thus, the specific gas concentration is accurately detectable.
Furthermore, it is preferable that the reference gas chamber for the second electrochemical cell is formed separately from the reference gas chamber for the first electrochemical cell.
According to this arrangement, even when the oxygen concentration in the reference gas chamber of the first electrochemical cell varies due to the pumping function, such variation is not transmitted to the reference gas chamber of the second electrochemical cell. Thus, the specific gas concentration is accurately detectable.
Furthermore, according to the present invention, it is preferable that the multilayered gas sensing element further comprises a heater incorporating a heat generating element capable of generating heat in response to current applied thereto, and an ion current path between the second electrochemical cell and the heat generating element is longer than an ion current path between the first electrochemical cell and the heater generating element.
In general, to assure accurate measurement of a specific gas concentration by the multilayered gas sensing element, it is necessary to quickly and sufficiently warm up the second electrochemical cell to its activation temperature. To this end, the multilayered gas sensing element is generally equipped with an electrically operable heater having a built-in heat generating element.
Electric power supplied to the heat generating element is very large compared with the sensor output level of the second electrochemical cell and therefore becomes a noise source giving adverse influence to the sensor output signal.
Electric power is also supplied to the first electrochemical cell. However, even if the current flowing across the first electrochemical cell contains a significant noise due to influence of power supply to the heat generating element, the output of the first electrochemical cell will not be so badly influenced because the first electrochemical cell is arranged to perform pumping of oxygen and the pumping performance is not so influenced by the current flowing therethrough.
Accordingly, to assure accurate measurement of the specific gas concentration, the ion current path between the second electrochemical cell and the heat generating element is set to be longer than the ion current path between the first electrochemical cell and the heater generating element.
The ion current path defined in this invention represents a current path along which the oxygen ion current can flow. Usually, the ion current path is a shortest path developed along an electrically conductive route, such as a solid electrolytic sheet, of the multilayered gas sensing element.
Furthermore, according to the present invention, it is preferable that the heater has a base material made of alumina.
Alumina is an insulating materia. The power current supplied to the heat generating element can be effectively prevented from flowing into other portion of the multilayered sensor element. Thus, the first and second electrochemical cells are not adversely influenced by the power current to the heat generating element. No noise is involved in the sensor output.
Furthermore, according to the present invention, it is preferable that an insulating resistance between the second electrochemical cell and the heat generating element is larger than an insulating resistance between the first electrochemical cell and the heat generating element. It is also preferable that a minium distance between the second electrochemical cell and the heat generating element is longer than a minimum distance between the first electrochemical cell and the heat generating element.
Electric power supplied to the heat generating element is very large compared with the sensor output level of the second electrochemical cell and therefore becomes a noise source giving adverse influence to the sensor output signal. On the other hand, even if the current flowing across the first electrochemical cell contains a significant noise due to influence of power supply to the heat generating element, the output of the first electrochemical cell will not be so badly influenced by the same reason described above.
Accordingly, to suppress the adverse influence of the heat generating element, the insulating resistance between the second electrochemical cell and the heat generating element is set to be larger than the insulating resistance between the first electrochemical cell and the heater generating element, thereby ensuring accurate measurement of the specific gas concentration,
From the similar reason, the minimum distance between the second electrochemical cell and the heat generating element is set to be longer than the minimum distance between the first electrochemical cell and the heater generating element, thereby ensuring accurate measurement of the specific gas concentration.
Furthermore, according to the present invention, it is preferable that the insulating resistance between the second electrochemical cell and the heat generating element is equal to or larger than 1xc3x9710xe2x88x9212 xcexa9, leak current between the second electrochemical cell and the heat generating element is equal to or smaller than 2xc3x9710 xe2x88x9211 A, and the minium distance between the second electrochemical cell and the heat generating element is equal to or larger than 0.4 mm.
Satisfying all of the above-described conditions will assure the accurate measurement of the specific gas concentration.
If the insulating resistance is smaller than 1xc3x971012 xcexa9, a significant leak current will flow from the heat generating element to the second electrochemical cell and therefore the output signal of the second electrochemical cell will contain a noise component. Thus, the sensor output will become inaccurate.
The leak current is a current flowing from the heat generating element to the second electrochemical cell. If the leak current exceeds 2xc3x9710xe2x88x9211 A, the output signal of the second electrochemical cell will contain a noise component. Thus, the sensor output will become inaccurate.
When the minium distance is smaller than 0.4 mm, the output signal of the second electrochemical cell will contain a noise component. Thus, the sensor output will become inaccurate.
It is preferable that an upper limit of the minimum distance is 10 mm. If the minimum distance exceeds 10 mm, a thermal capacity of the sensor element will become so large that it will take a long time for each electrochemical cell to reach an active temperature after activation of the heater.
Furthermore, according to the present invention, it is preferable that the first electrochemical cell comprises a pair of electrodes provided on opposite surfaces of a first solid electrolytic sheet, and the second electrochemical cell comprises a pair of electrodes provided on opposite surfaces of a second solid electrolytic sheet which is different from the first solid electrolytic sheet.
The voltage applied to the first electrochemical cell is larger than the output level of the second electrochemical cell. Thus, to reduce or eliminate adverse influence of the voltage applied to the first electrochemical cell, the second electrochemical cell is provided on the solid electrolytic sheet different and spaced from the solid electrolytic sheet of the first electrochemical cell, thereby ensuring accurate measurement of the specific gas concentration.
Furthermore, according to the present invention, it is preferable that an alumina sheet is disposed between the first electrochemical cell and the second electrochemical cell. This arrangement is effective to prevent the leak current from flowing from the first electrochemical cell to the second electrochemical cell or vice versa, thereby ensure the accurate detection of the specific gas concentration.
Furthermore, according to the present invention, it is preferable that a third electrochemical cell is provided to measure an oxygen concentration in one of the first chamber and the second chamber.
In this case, in addition to measurement of the specific gas concentration (e.g., NOx concentration), the oxygen concentration is measured in either the first chamber or the second chambers. Thus, it becomes possible to maintain the oxygen concentration in at least one of first and second chambers to a constant level. Hence, the measuring accuracy of the specific gas concentration by the second electrochemical cell can be further ensured.